A typical flyback converter includes a primary side circuit, a transformer, and a secondary side circuit. The primary side circuit is connected to a power source and includes at least one switching element that controls the amount of energy transferred to the secondary side via the transformer. The transformer serves as an electrically isolated channel to transfer energy from the primary side circuit to the secondary side circuit. The secondary side circuit is coupled to a load to be powered.
In a traditional flyback converter, at least one diode coupled in a current path of a secondary side winding of the transformer is included to block current (e.g., from flowing from the transformer to the secondary side circuit when the primary side transistor is turned on or from flowing from an output capacitor on the secondary side to the secondary side winding and back to the primary side). One disadvantage of using a diode in the secondary side circuit is that, when the primary side switching element is turned off and energy is transferred from the transformer to the secondary side circuit (and the load), energy is lost due to a voltage drop (RDS-ON) over the diode. To improve efficiency, some flyback converters may be configured such that the traditional diode is replaced by, or put in parallel with, an active element (e.g., one or more transistors), which may be referred to as a secondary side switching element. Such a secondary side switching element may be operated to switch in synchronization with switching behavior of the primary side switching element, which may increase efficiency compared to the using a diode as described above. Operation of the secondary side switching element in synchronization with switching behavior of the primary side switching element may be referred to as synchronous rectification. Generally, there are two ways to implement synchronous rectification: the first way is referred to as “control-driven” synchronous rectification, and the second way is known as “self-driven” synchronous rectification.
In a control-driven scheme, the secondary side switching element is driven by gate-drive signals that are derived from the gate-drive signal of the primary side switching element. In other words, the control-driven scheme generally requires information to pass, via one or more additional electrically isolated signal paths or communication links other than the transformer, from a primary side circuit of the flyback to a secondary side circuit of the flyback. Using the information received via the additional electrically isolated signal paths, sent from the primary side, a secondary side controller can determine the state of the gate-drive signals controlling the primary side switching element. Based on the state of the gate-drive signals controlling the primary side switching element, the secondary side circuit can determine when to cause the secondary side switching element to turn-on or turn-off in synchronization with the primary side switching element. Since a control-driven synchronous rectification control scheme uses an additional, communication link, control-driven synchronous rectification may increase size, cost, and/or complexity of the flyback power converter.
Self-driven synchronous rectification may be more attractive for some flyback applications since self-driven control is simpler and requires fewer components than the control driven scheme. In a self-driven scheme, a secondary side controller may forgo the information about the state of the gate-drive signals controlling the primary side switching element, received from the primary side circuit via the additional, communication link, and instead may simply monitor energy (e.g., a current and/or voltage of energy) being transmitted to the secondary side circuit via the transformer. Based on the monitored energy, the secondary side controller can control the secondary side switching element to switch in-synchronization with the operations of the primary side switching element. Although the reliance on a self-driven synchronous rectification control scheme may decrease size, cost, and/or complexity as compared to a control-driven scheme, self-driven synchronous rectification may sacrifice accuracy and quality of a flyback converter by producing a lower quality and less efficient power output.